FIG. 2 shows a cross-sectional view of a liquid crystal display having a thin film transistor for controlling the display state of each pixel. In FIG. 2, reference numeral 1 designates a liquid crystal display comprising a plurality of pixels and reference numeral 10 designates a thin film transistor produced on the rear substrate 100 of the liquid crystal display 1. Reference numeral 200 designates a transparent front substrate and 210 is a color filter disposed at the internal surface of the substrate 200. Reference numeral 220 designates a transparent electrode mounted on the color filter 210. Reference numeral 300 designates a seal disposed between the two opposed substrates 100 and 200. Reference numeral 400 designates liquid crystal material between the substrates 100 and 200.
FIG. 3 shows a cross-sectional structure of the thin film transistor 10 In FIG. 3, reference numeral 11 designates a semi-insulating substrate comprising a material such as glass. Reference numeral 12 designates a gate electrode comprising a material such as Cr disposed on the insulating substrate 11. Reference numeral 13 designates a gate insulating layer comprising a material such as SiN or SiO.sub.2 disposed on the gate electrode 12. Reference numeral 14 designates a semiconductor layer comprising amorphous silicon, disposed on the gate insulating layer 13. Reference numerals 15 and 16 designate a source electrode and a drain electrode, respectively, comprising Al or Cr disposed on the semiconductor layer 14. Reference numeral 17 designates a protection film comprising an insulating material such as SiN or SiO.sub.2 disposed covering the entire surface.
FIG. 5 is a diagram for explaining the operation of the thin film transistor 10 included in the liquid crystal display 1. In FIG. 5, the source electrode 15 is insulated from the liquid crystal material 400 and the drain electrode 16 is in contact with the liquid crystal material 400.
When a constant DC voltage E.sub.1 is applied between the source electrode 15 and the drain electrode 16, negative charges in the semiconductor layer 14 flow from the drain electrode 16 to the source electrode 15. Even when a variable gate voltage E.sub.2 is applied between the source electrode 15 and the gate electrode 12, the semiconductor layer 14 does not affect the flow of the negative charges so long as this gate voltage E.sub.2 is low. When the gate voltage E.sub.2 is increased to exceed a threshold voltage E.sub.t, the negative charges of the semiconductor layer 14 are pulled toward the gate electrode 12 and, with an increase in the gate voltage E.sub.2, the drain current I.sub.d flowing between the gate electrode 12 and the drain electrode 16 rapidly increases and is finally saturated because the charge in the semiconductor layer 14 is constant. The relationship between the drain current I.sub.d flowing between the gate electrode 12, the drain electrode 16, and the gate voltage E.sub.2 is as shown in FIG. 12.
Herein, the DC voltage E.sub.2 +E.sub.3 which is applied between the transparent electrode 220 which is fixed to the color filter 210 of the front substrate 200 of the liquid crystal display 1 and the gate electrode 12 is essentially applied to the gate insulating layer 13 of the thin film transistor 10 when the gate voltage E.sub.2 is low. On the other hand, when the gate voltage E.sub.2 is increased above the threshold voltage E.sub.t, as described above, the drain current I.sub.d flows between the gate electrode 12 and the drain electrode 16, the apparent resistance between the gate electrode 12 and the drain electrode 16 is lowered, and the DC voltage E.sub.2 +E.sub.3 is applied to the liquid crystal material 400 between the transparent electrode 220 and the drain electrode 16. As a result, the molecules of the liquid crystal material 400 are arranged in the direction of the electric field produced by the DC voltage E.sub.2 +E.sub.3 and become transparent to the transmitted light or incident light.
Then, when the gate voltage E.sub.2 is reduced, an operation reverse to the above-described one is obtained, and the degree of orientation of the molecules of the liquid crystal material 400 is reduced and the liquid crystal material 400 becomes opaque to the transmitted light or incident light. In this way, through the switching function of the plurality of thin film transistors 10 in the liquid crystal display, the display state of the liquid crystal display is controlled.
When, during operation, the gate voltage 14 is altered by static electricity from friction between the rear substrate 10 and the front substrate 200 with the other materials or an external voltage surge is applied between the gate electrode 12 and the source electrode 15 of the thin film transistor 10, positive or negative charges are injected from the semiconductor layer 14 into the gate insulating film 13. These charges are captured by traps in the gate insulating layer 13 in the neighborhood of the interface between the gate insulating film 13 and the semiconductor layer 14, with the result that a space charge is generated in the gate insulating layer 13.
When the voltage produced by the space charge is opposite that of the gate voltage E.sub.2 of the thin film transistor 10, the relationship between the drain current I.sub.d and the gate voltage E.sub.2 shown in FIG. 12 drifts toward the abnormal state (curve a) which has a higher threshold than normal (curve b). Then, instead of having the highest brightness at the gate voltage E.sub.0, a pixel for which the above-described relation between I.sub.d and E.sub.2 of the thin film transistor 10 has drifted to the curve a has a low brightness.
On the contrary, when the direction of the voltage produced by the space charge has the same direction as the gate voltage E.sub.2 of the thin film transistor 10, the threshold value drifts to the lower side, from the normal threshold voltage E.sub.b (curve b) to the low threshold voltage E.sub.c (curve c). Then, a normal pixel with a thin film transistor receiving a gate voltage E.sub.0 and an abnormal pixel in which the I.sub.d -E.sub.2 relation of the associated thin film transistor 10 has drifted toward the curve c both have the maximum brightness. Even when the brightness of the normal pixel is made darker by lowering the driving voltage from E.sub.2 to E.sub.b, the abnormal pixel remains bright.
Variations in the threshold voltages of several tens of thousands to several hundreds of thousands of thin film transistors included in a liquid crystal display appear as a line defect or a point defect on the liquid crystal display screen.
In order to suppress the drift of I.sub.d -E.sub.2 of the thin film transistor 10, as discussed in "Japan Society for the Promotion of Science, Amorphous Material 147th Committee 15th Conference Material, p 12 to p 15 (Feb. 17, 1987)", the composition of the gas mixture, such as the gas ratio of N.sub.2 to NH.sub.3, while producing the gate insulating layer 13, is stoichiometrically optimized, the production temperature is raised to 300.degree. to 360.degree. C. from the conventional temperature of approximately 200.degree. C. or the threshold voltage drift is made uniform by annealing for 1 to 3 hours at a temperature of 200.degree. to 400.degree. C. after producing the thin film transistor 10. In all the above-described conventional methods, the number of traps in the gate insulating layer 13 is reduced. Other than that, charges captured in the traps at the gate insulating layer 13 may be released by applying thermal energy or optical energy to the thin film transistor 10.
However, when the conventional methods of suppressing the shift of the I.sub.d - E.sub.2 relationship of the thin film transistor 10 are utilized, the number of traps at the gate insulating layer 13 of the thin film transistor 10 is reduced, but the traps cannot be completely eliminated. Therefore, when an external electric field is again applied to the thin film transistor 10, there is the possibility that a shift in the I.sub.d - E.sub.2 relationship of the thin film transistor will occur.
Furthermore, the liquid crystal display 1 may produce a faulty display due to the static electricity between the rear substrate 100 and the front substrate 200 of the liquid crystal display 1 or an external surge voltage, making it necessary to restore the liquid crystal display 1.
However, because the liquid crystal material of the liquid crystal material 1 is likely to be resolved by heat, high temperature annealing is not used. Furthermore, although it is possible to restore the faulty display by irradiating the thin film transistor 10 corresponding to the pixel showing the faulty display with light, if too much light energy is applied, the liquid crystal material 400 is likely to be resolved or the characteristics of the semiconductor layer 14 are likely to be changed. On the contrary, when the supply of the light energy is insufficient, the restoration of the faulty display is unsatisfactory.